Thermal deposition of metals in a vacuum



Oct. 30, 1945. P, ALEXANDER 2,387,970

THERMAL DEPOSITION OF METALS IN A VACUUM Filed July 18, 11942 /nT/I/l 111 47m/MEM denum.

i Patented ocr. 3,0, 1945 THERMAL DEPOSITIONOF METALS IN A VACUUM Paul Alexander,`erkhamsted, England Application July 18, 1942, Serial No. 451,468 In Great Britain September 16, 1941 s claims. (ci. 117-107) This invention relates to the thermal deposition of metals in a vacuum.

In the process of thermal deposition in a vacuum, the metal to be deposited is heated to a temperature above its vaporisation point in a vacuum which may be from 0.00001 to 0.01 mm. Hg, depending on the process employed. The metal may be heated by a metallic heater or it may be heated in a non-metallic container. When a metallic heater is employed, the metal used for the heater must have a negligibly small vapour pressure at the temperature towhich it is heated, so that no appreciable amount of this metal is vaporised to contaminate the vapour of the metal to be deposited.

Now the metals commonly required to be deposited are aluminium,copper, silver, gold and nickel, which require to be heated to 1000 degrees C. or over for their vaporisation. For metals 1, `with vaporisation temperatures of this order, the only metals available for use as heaters are the refractory metals, such as-tungsten and molyb- Certain metals, and, in particular, aluminium, dissolve, or alloy with, the refractorymetals at the temperatures employed, to a small extent, but enough to make them mechanically weak. Thus when, for instance,

i, aluminium is brought into contact with the heater metal at a temperature. adapted to the vaporisation of the aluminium, the heater breaks time in breaking and re-establishing the vacuum,-

due to the metal to be evaporated alloying with and destroying the heater, by the use of a liquid heater with which the metal is alloyed or mixed.

Accordingly the metal to be deposited may be fed tothe liquid heater as a wire or ribbon, as

` in the case of aluminium, or may be fed to the A metal into the liquid, as in the case of chromium, I

after a short time and has to be replaced. The

life of a heater of the form commonly' employed for vaporising aluminium is from ten to twenty minutes. The replacement of heaters entails reevacuation of the chamber, which requires from half-an-hour to several hours, depending on the degree of vacuum employed. The process, therefore, is intermittent, and the interruptions entail much loss of working time.

However, in the specification of Patent No. 2,153,786 the applicant herein, described and claimed a process of thermal deposition in a vacuum of a metal to be evaporated in which this metal is fed asa wire or rlbbon,end on, to a heater at such a rate that the metal is evaporated substantially as soon as it comes into contact with the heater and thereby the metal cannot lie on the heater in a molten state, thus. alloying is avoided and a continuous process thereby obtained, whilst loss of time in breaking and re-establishing the vacuum to eect repairs is entirely eliminated. f

The main object of the present inventionis to provide an improved continuous process of there mal evaporation of metals in which the vapour mixture by dropping pellets or crystals of the which metal cannot be drawn into wire form.

According to the present invention a process of depositing on a support a bright illm of a metal selected from the group consisting of gold,- nickel, cobalt, iron, copper. aluminium, chromium, and silver, which is deposited by thermal evaporation in a vacuum consists in continuously maintaining liquid a mixture of a metal, selected from the group consisting of platinum and palladium and a small percentage of the said metal to be evaporated. exposing said support to the vapour'from said mixture, and maintaining th'e percentage of saidmetal to be evaporated approximately constant in said mixture by feeding the metal to be evaporated into the said mixture at a rate equal to the rate of evaporation of said metal to be evaporated.

Apparatus for carrying out the process according to the invention comprises in a vacuum chamber, a heater consisting of a bath of liquid platinum or palladium, a vessel adapted to contain the liquid bath and means for maintaining the platinum or palladium liquid, and means for feeding into the liquid bath the metal to be evaporated at a rate adjustableto maintain an approximately constant and small percentage of the metal to be evaporated in the liquid bath.

In a preferred form of apparatus according to the invention the vessel increases in cross section at its ends and the mixture is heated by electrodes of refractory metal, preferably tungsten, 4entering of the vessel, the cross section of the vessel where the container metal makes contact with theeiectrodes being sumcient to pass the heating current without melting, while the metals in the central portion of the bath are liquid.

Figure 1 is a vertical section showing a vacuum chamber with thermal deposition appa.-

ratus;

Figure 2 is a vertical section to an enlarged scale of the non-metallic heating vessel shown in Figure l;

Figure 3 is a plan of a modilled form of the heating vessel of which Figure 4 is a vertical section; and

Figure 5 is a sectional elevation of a pellet or powder dropping device.

In the drawing like references designate the same or similar parts.

Referring first to Figures 1 and 2 of the drawing, the vacuum chamber I contains the support 2 lon which the metal is to be deposited, a liquid mixture 3 of platinum or palladium contained in a refractory vessel 4, for example of alumina, and a, clockwork 5 adapted to feed at an adjustable rate a wire 6 of the metal to be deposited from'a spool 1 into the bath 3. A screen 8 shields the neighbourhood of the point oi' contact between the wire 6 and the liquid-3, so as to protect the support from the denser vapour issuing from this point. Figure 2 shows, to an v enlarged scale, the bath 3 of platinum or palladium contained in a vessel 4 o f refractory ma4 terial, such as alumina. Lugs 9 serve to support the vessel 4 on the brackets I0 (Figure 1)'. The bath is heated by a zig-zag coil Il of refractory metal, such as tungsten, embedded in the refractory material I2 of the vessel 4 so that the coil is protected from contact with the molten metal and shielded from the vapour ofthe metal evaporated.

Figures 3 and 4 show an alternative form ot heating vessel, in which th'e cross-section of the bath at the ends I3 is considerably larger than in the central channel and electrodes I4 of rei'ractorymetal enter the bath at the ends I3'A so that the metal of the bath is partly liquid and partly solid.

Heating current is supplied through the electrodes I4 to heat the bath and the cross section of the bath at the ends I3 is sufiicient to pass the heating current without melting the'metal at the ends whilst the platinum or palladium in the relatively narrow central channel is liquid. As the metal at the ends is solid the electrodes I4 are at a temperature below that at which'the vapour of the metal being deposited can alloy with them.

By forming the electrodes and if desired, also the ends of the bath with relatively large heat radiating surfaces, the temperature in the region of the electrodes, for a given current density,1may be lowered, whereby minimum cross section of the ends of the bath may be attained, while still maintaining the metal in the ends of the bath in the solid state.

Figure 5 shows a device for dropping pellets or powder of the metal into the bath through the screen 8. The wheel I5, driven slowly by a clockwork similar to 5, is notched, and takes pellets or powder from the hopper I8 and, as it turns, drops the pellets or powder into the bath at the predetermined rate of feed.

A small proportion of' the metal to be evaporated is mixed with platinum, a convenient proportion being from three per cent to twenty per cent. 'I'he mixture is heated in the vessel 4 and maintained in a liquid condition, with liquid surface exposed to the support 2, for example made of glass or paper on which the metal is to be deposited.

Assuming, for example, that aluminium is be- `ing evaporated, and that a platinum-aluminium mixture containing about 7% aluminium is heated to about 1700 degrees C., the vapour pressure of the aluminium is then about 10 mm., and that of the platinum is about 0.0001 mm. oL-Hg. There is, therefore, no appreciable evaporation of the platinum, while the aluminium evaporates readily in a vacuum of 0.01 mm. of Hg. Since the proportion of aluminium to platinum is small, the vapour density of the aluminium at the surface of the liquid is low. The process is made continuous by feeding aluminium into the liquid mixture at a rate approximately equal to the rate at which the aluminium is evaporated, so that the mixture is maintained at approximately constant composition. The rate of feedl is such as to ensure the aluminium llquifying and diffusing in the platinum without appreciable vaporisation.

'I'he liquid mixture in the bath is kept substantially homogeneous by diffusion and by the convection currents in the liquid, apart from the region of somewhat higher concentration in the neighbourhood of the feeding point. The screen 8 serves also to prevent the vapour from this region from reaching the support 2.

By increasing the proportion in the liquid mix- 40 ture of the metal to be evaporated, the rate of deposit is increased, but,V at the same time, the vapour density is increased. It is, therefore, of advantage to increase the proportion, and so secure rapid working, while keeping the propor-` tion below that which gives a dull deposit at the vacuum employed.

This adjustment, to obtain optimum working conditions in any given installation, can be made by adjusting therate of feed of metal into the liquid mixture.

Theword mixture has been employed to denote the combination of the metal to be deposited with the platinum. Since the mixture is operative only when liquid, it is immaterial whether one metal is alloyed with the other, or dissolved in itor merely mixed with it, and the word mixture" is used to denote any one of the forms in which the metals can be combined when in liquid form'.

'The invention has the further advantage that the vapour density of the metal vapour can be kept as low as desired by adjusting the proportion of the metal to be evaporated in the bath. It is known that a illm of good quality, having a bright surface, can be deposited only in a high degree of vacuum, of 0.001 to 0.00001 mm. Hg if the vapour density of the metal is high, and in theabsence' of special heating means for the vapour, but that, if the vapour density is kept low, a, vacuum of only 0.01 to 0.001fmm. Hg is sumcient to obtain a. bright nlm, l

'Ihe following metals, among others, may be evaporated by the process thevsubject of the invention: gold, nickel, cobalt, iron, copper, aluminium, chromium, silver. In the case of chromium, which cannot be drawn into wire, the feeding is effected by dropping pellets or crystals of the metal into the liquid.

Either platinum or palladium may be used as the metal with which the metal to be evaporated is mixed.

From the foregoing it will be observed that the invention comprises the operation of a continuous process for the thermal deposition of a metal from a liquid mixture comprising platinum or palladium, within which has been incorporated a small percentage of the'metal to be evaporated. and the invention also covers the coated support produced in carrying out the process herein described.

I claim:

1. In a process of depositing on a support a bright film of a metal. selected from the group consisting of gold, nickel, cobalt, iron, copper, aluminium, chromium and silver, which is deposited by thermal evaporation in a vacuum, the steps of continuously maintaining liquid a mixture of a metal, selected from the group consisting of platinum and palladium, and a small percent'age of the said metal to be evaporated. exposing said support to the vapour from said mixture, and maintaining the percentage of said metal to be evaporated approximately constant in said mixture by feeding the metal to be evaporated into the said mixture at a rate equal to the rate of evaporation of said metal to be evaporated.

2. A process according to claim 1 including the steps of proportioning the said metal to be evaporated to the metal selected from the 4group consisting of platinum and palladium in such a manner as to obtain a low vapour density of the said mixture, and evaporating the said mixture in a vacuum of 0.01 to 0.001 mm. Hg.

3. A process according to claim l, wherein said mixture contains a proportion of from three to twenty per cent of said metal to be evaporated.

4, A process according to claim 1, wherein the said metal to be evaporated is aluminium, the said mixture containing about '1% thereof.

5. Apparatus forthe thermal evaporation in a vacuum of metals of high vaporisation point, comprising a vacuum chamber, a heater in the chamber consisting of a bath of a liquid of a metal selected from the group consisting of platinum and palladium. a vessel adapted to contain the liquid bath" and means for maintaining the bath liquid, and means for feeding into the liquid bath the metal to be evaporated at a rate adjustable to maintain an approximately constant and small percentage of the metal to be evaporated in the liquid bath. v

6. Apparatus for the thermal evaporation in a vacuum of metals of high vaporisation point, comprising a vacuum chamber, a, heater in the chamber consisting oi' a bath of a liquid of a metal selected from the group consisting of platinum and palladium, a vessel adapted to contain the liquid bath, the vessel being of increased cross section at its ends, and heating electrodes of refractory metal entering the said ends of said vessel, the cross section of the vessel at the ends thereof being sufficient to enable a heating current supplied to the electrodes to pass through the metal in the said ends Without melting it. whilst the metal intermediate the ends of the vessel is melted to form the liquid bath, and means for feeding into the bath the metal to be evaporated. at a rate adjustable to maintain an approximately constant and small percentage of the metal to be evaporated in the liquid bath.

7. Apparatus for the thermal evaporation in a vacuum of metals of high vaporisation point. comprising arvacuum chamber, a vessel of nonmetallic material, a quantity of a metal selected from the group consisting of platinum and palladium in said vessel. means for continuously maintaining at least a portion of said quantityof metal liquid, whereby a bath of the liquid is contained in said vessel and constitutes a heater for the metal to be evaporated, means for feeding to the surface of the bath a length of wire of the metal to be evaporated and means for regulating the rate of feed of the wire to maintain an approximately constant and small percentage of the metal to be evaporated in the liquid bath.

8. Apparatus for the thermal evaporation in a vacuum of metals of high vaporisation point, 'comprising a vacuum chamber, a vessel of non-metallic material, a quantity of a metal selected from the group consisting of platinum and palladium in said vessel, means for continuously maintaining at least a portion of said quantity of metal liquid, whereby a bath of the liquid is contained in said vessel and constitutes a heater for the metal to be evaporated, said means including an electrically heated coil of refractory material em-` bedded within the material of the vessel, and means for feeding into the liquid the metal. to be evaporated at a rate adjustable to maintain an approximately constant and small percentage of l the metal to be evaporated in the liquid bath.

PAUL ALEXANDER.

CERTIFICATE OF CORRECTION.

Patent No. 2,587,970. October 50, l9h5.

PAUL ALEXANDER t isV hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, second column, line 66, for "O.OO1n read --OOO1; and that the said Letters Patent should be read with this correction therein that the same may confom to the record of the case in the Patent Office,

Signed and sealed this 26th' day of February, A., D. 191%.

Leslie Frazer (Seal) First Assis tant Commissioner of Patents. 

